Peroxidase enzymes such as horseradish peroxidase (HRP) are frequently used as markers or labels in enzyme-linked assays for biological molecules and other analytes of interest such as drugs, hormones, steroids and cancer markers. Chemiluminescent detection of these enzymes offers a safe, convenient and sensitive means of measuring the amount of enzyme in a sample or the amount of an enzyme-labeled analyte or labeled specific binding partner for an analyte. Other chemiluminescent reaction schemes have been developed to quantify the level of particular peroxidase enzymes.
a. Chemiluminescent Peroxidase Substrates. Amino-substituted cyclic acylhydrazides such as the well-known luminol and isoluminol react with H2O2 and a peroxidase catalyst (such as horseradish peroxidase, HRP) under basic conditions with emission of light. This reaction has been used as the basis for analytical methods for the detection of H2O2 and for the peroxidase. Heterocyclic analogs of luminol such as 8-amino-5-chloro-7-phenylpyrido[3,4-d-pyridazine-1,4(2H,3H)dione (M. Ii, et al., Biochem. Biophys. Res. Comm., 193(2), 540-5 (1993)); pyridazinoquinoxalinones (U.S. Pat. No. 5,324,835) and 1,3-disubstituted pyrazolo[4′,3′:5′,6′]pyrido-[2,3-d]-pyrazinediones (Y. Tominaga, et al., Tetrahedron Lett., 36, 8641-4 (1995)) are known to react with a peroxidase and peroxide to produce chemiluminescence. Other hydrazide compounds which are chemiluminescent when oxidized by a peroxidase and a peroxide are hydroxy-substituted phthalhydrazides (U.S. Pat. No. 5,552,298).
Applicant's U.S. Pat. Nos. 5,491,072, 5,523,212 and 5,593,845 disclose chemiluminescent N-alkylacridan-carboxylic acid esters, thioesters and sulfonimides which produce light upon reaction with a peroxide and a peroxidase for use in detecting peroxidases and in assays. A PCT application (WO 94/02486) describes the chemiluminescent reaction of spiroacridan compounds with hydrogen peroxide. The reaction is enhanced by the addition of horseradish peroxidase.
Various compounds of biological origin, collectively termed luciferins, are oxidized by a peroxidase (summarized in L. J Kricka and G. H. G. Thorpe, in Luminescence Immunoassay and Molecular Applications, K. Van Dyke and R. Van Dyke, eds., CRC Press, Boca Raton, 1990, pp. 77-98). When hydrogen peroxide is not utilized, the enzyme is functioning as an oxidase.
Certain phenol compounds produce chemiluminescence on oxidation with a peroxidase. As examples, pyrogallol B-1 and purpurogallin B-2 are cited in Kricka and Thorpe, ibid. as well as the coumarin-type compounds coumarin, umbelliferone and esculin (D. Slawinska, J. Slowinski, J. Biolumin. Chemilumin., 4, 226-30 (1989)); phloroglucinol B-3 (M. Halmann, et al., Photochem. Photobiol., 30, 165-7 (1979)); and acetaminophen B-4 (K. Schmitt, G. Cilento, Photochem. Photobiol., 51, 719-23 (1990)).

Other miscellaneous compounds reported to produce weak chemiluminescence in the presence of a oxygen or peroxide and a peroxidase are a synthetic Schiff base-containing polymer ((R. Zoulik, et al., Coll. Czech. Chem. Commun., 60, 95-103 (1995)); indole-3-acetic acid in the presence of xanthene dyes with or without hydrogen peroxide (S. Krylov, A. Chebotareva, FEBS, 324(1), 6-8 (1993); tyrosine, tryptophan and chlorpromazine (M. Nakano, J. Biolumin. Chemilumin. 4, 231-40 (1989)) and MCLA B-8 M. (Mitani, et al., J. Biolumin. Chemilumin. 9, 355-61 (1994)) which have the respective structures B-5-B-8 as shown below.
None of the foregoing references disclose the chemiluminescent oxidation of the presently disclosed compounds by a peroxidase
b. Reaction of Enols with HRP. A series of papers describe the peroxidase-catalyzed air oxidation of enolizable aldehydes (H. Gallardo, et al., Biochim. Biophys. Acta, 789, 57-62 (1984); W. J. Baader, et al., Biochem. Ed., 14(4), 190-2 (1986); I. Nantes, et al., Photochem. Photobiol., 63(6), 702-8 (1996)). The reactive substrate is thought to be the small quantity of the enol form in equilibrium with the aldehyde. The reaction of the aldehyde is catalyzed by enol phosphates, but the enol phosphate itself is not consumed. The reference teaches that the enol phosphate does not react with a peroxidase to produce chemiluminescence. Energy transfer to fluorescent energy acceptors increased light emission (M. T. Grijalba, et al., Photochem. Photobiol., 63(6), 697-701 (1996)). Aldehydes masked as enol silyl ethers (Baader, ibid.) or enol acetates were used in coupled assays in which the enol was unmasked in a first step to generate an enol in situ which subsequently reacted with a peroxidase to generate chemiluminescence (A. Campa, et al., Photochem. Photobiol., 63(6), 742-5 (1996)).
c. Peroxidase Enhancers. Numerous enhancers have been employed in order to increase the quantity and duration of chemiluminescence from the reaction of a peroxidases with known chemiluminescent substrates including the aforementioned luminol and the acridancarboxylic acid derivatives. These include benzothiazole derivatives such as D-luciferin, various phenolic compounds such as p-iodophenol, p-phenylphenol, naphthols and aromatic amines as listed in G. Thorpe, L. Kricka, in Bioluminescence and Chemiluminescence, New Perspectives, J. Scholmerich, et al, Eds., pp. 199-208 (1987). Other compounds which function as enhancers of the chemiluminescent oxidation of amino-substituted cyclic acylhydrazides by a peroxidase include 4-(4-hydroxyphenyl)-thiazole (M. Ii, ibid.), a group of compounds disclosed in U.S. Pat. No. 5,171,668, 2-hydroxy-9-fluorenone, and a group of hydroxy-substituted benzoxazole derivatives as disclosed in U.S. Pat. No. 5,206,149 and certain phenylboronic acid compounds as described in U.S. Pat. No. 5,629,168. None of the foregoing references disclose the chemiluminescent oxidation of the present compounds by a peroxidase alone or with the use of enhancers.
d. Enhancement of Chemiluminescent Peroxidase Reactions by Surfactants. Enhancement of the chemiluminescence produced in peroxidase-catalyzed reactions using polymeric and monomeric surfactants is known in the art. Enhancement can occur by affecting the outcome of one or more steps e.g. by increasing the fluorescence quantum yield of the emitter, by increasing the percentage of product molecules produced in the excited state, by increasing the fraction of molecules undergoing the chemiluminescent reaction through inhibition of competing side reactions or by promoting the action of an enzyme catalyst. No clear or consistent pattern exists concerning the effect of polymeric and monomeric surfactants on chemiluminescent reactions. It is impossible to predict which surfactant compounds, if any, may enhance the chemiluminescence from a particular process and can only be determined by substantial experimentation.
The cationic polymeric surfactant poly-N-ethyl-4-vinyl-pyridinium bromide completely inhibited the chemiluminescent reaction of luminol by a negatively charged insulin-peroxidase conjugate and diminished chemiluminescence to a lesser extent when the native enzyme was used (S. B. Vlasenko, et al., J. Biolumin. Chemilumin., 4, 164-176 (1989)).
A published Japanese Patent Application No. JP 06,242,111 and a paper (R. Iwata, et al., Anal. Biochem., 231, 170-4 (1995)) disclose the use of nonionic surfactant and skim milk in the chemiluminescent peroxidation of luminol to lower background emission or enhance signal/noise.
None of the foregoing references disclose the chemiluminescent oxidation of the present compounds by a peroxidase or chemiluminescence enhancement with surfactants.
e. Assays using HRP. The enzyme horseradish peroxidase has found widespread use in enzyme immunoassays and DNA hybridization assays with chemiluminescent detection using luminol or isoluminol as substrate. Commercially available kits using HRP conjugates and enhanced luminol chemiluminescent detection are available. Chemiluminescent peroxidase assays are also disclosed in the aforementioned U.S. Pat. Nos. 5,491,072, 5,523,212 and 5,593,845. No references disclose the chemiluminescent peroxidase assays using the present compounds as the substrates.
f. New Chemiluminescent Peroxidase Substrates. A new class of chemiluminescent peroxidase substrates was disclosed by Applicants in their previous U.S. Pat. No. 5,922,558, and published PCT Application No. WO99/14220. The disclosure of these applications is fully incorporated herein. While the disclosure of these publications describes a class of heterocyclic compounds generic of the compounds of the present invention, the present compounds are unexpectedly superior in producing light from reaction with a peroxidase enzyme and a source of peroxide.
g. Benzodithiafulvalenes. Synthesis of a benzodithiafulvalene having the structure below has been described. (K. Akiba, K. Ishikawa, N. Inamoto, Synthesis, 12, 861-2 (1977). No information concerning its ability to function as a chemiluminescent peroxidase substrate is known.
